Hydrogen gas sensing performance of a carbon-doped boron nitride nanoribbon at elevated temperatures

In this study, computational simulations were used to investigate the performance of a carbon-doped boron nitride nanoribbon (BC2NNR) for hydrogen (H2) gas sensing at elevated temperatures. The adsorption energy and charge transfer were calculated when H2 was simultaneously attached to carbon, boron, and both boron and nitrogen atoms. The sensing ability was further analyzed considering the variations in current–voltage (I–V) characteristics. The simulation results indicated that the energy bandgap of H2 on carbon, boron, and both boron and nitrogen exhibited a marginal effect during temperature variations. However, significant differences were observed in terms of adsorption energy at a temperature of 500 K, wherein the adsorption energy was increased by 99.62% of that observed at 298 K. Additionally, the evaluation of charge transfer indicated that the strongest binding site was achieved at high adsorption energies with high charge transfers. Analysis of the I–V characteristics verified that the currents were considerably affected, particularly when a certain concentration of H2 molecules was added at the highest sensitivity of 15.02% with a bias voltage of 3 V. The sensitivity at 298 K was lower than those observed at 500 and 1000 K. The study findings can form the basis for further experimental investigations on BC2NNR as a hydrogen sensor.


Introduction
Gas sensors are extremely essential in today's society, notably in industrial production and public safety [1]. For detecting various hazardous gases, a high-performance solid-state gas sensor with high sensitivity and selectivity is preferred. Maintaining the stability and selectivity of gas sensors, particularly at higher temperatures, is a challenge in sensing applications [2]. Ideally, high sensitivity, high selectivity, fast response, and rapid recovery time are the desired properties for effective sensing [3,4].
Metal-based gas sensors were introduced in early 1962; subsequently, Zhang et al. [1] reported different metal oxides involving CeO 2 , ZnO, SnO 2 , and CuO as active materials for a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 As much as previous studied has done based on experimental works, there is also studies carried in simulation works. For examples, a first-principles study on hydrogen gas sensing of carbon-doped ZnO nanotube by Rezaie et al. [21]. It was found, hydrogen gas is chemically absorbed to the carbon atoms and shows high response with sensitivity of 73% at V = 3.5 V on the outside and inside of carbon-doped ZnO nanotube. Another simulation works focusing on hydrogen gas binding energies on carbon-doped BN nanotube demonstrated by Baierle et al. [22]. Subsequently, the binding energies reported to have similar value of −156 meV and −163 meV for H 2 molecule absorbed on BNNT + C B and BNNT + C N , respectively. Moreover, a donor and acceptor defect level were shown when H 2 molecule absorbed on BNNT + C B and BNNT + C N , respectively. Apart from that, Yu et al. [23] studied the potential of monolayer Ti 2 CO 2 as gas sensor in detecting NH 3 , H 2 , CH 4 , CO, CO 2 , N 2 , NO 2 and O 2 gas. Among the gases tested, NH 3 was found chemisorbed and have the high sensitivity and selectivity with highest adsorption energy of −0.37 eV. By applying 3% strains on Ti 2 CO 2 , NH 3 have the high sensitivity and selectivity with highest adsorption energy of −0.51 eV. Another successfully simulation work performed by Zhou et al. [24], studied the monolayer WS 2 upon adsorption and the impacts of charge transfer mechanism on Fermi-level pinning in sensing H 2 , O 2 , H 2 O, NH 3 , NO, NO 2 and CO gases. Through this, O 2 , H 2 O, NH 3 and NO 2 gases were seen to be a promising candidate as a sensor to WS 2 with moderate adsorption energies and charge transfer.
BN is known for certain properties, such as a wide band gap of approximately 5−6 eV, excellent mechanical strength, high thermo-conductivity, high thermal stability, appropriate chemical inertness, adequate oxidation stability (up to 800˚C), and suitable optical properties [25]. Numerous studies have used BN for sensing applications. According to Sajjad et al. [6] that carbon-based band gap tailoring approaches significantly enhance the band gap. The alloying process involving C, B, and N atoms has led to the formation of certain configurations, such as hexagonal boron carbonitride (BxCyNz) and BCN. Based on our literature review, we found number of works has been done investigating nanomaterial for use in hydrogen gas sensing application as in reference [15,22]. The emerging of nanomaterial such as graphene, carbon nanotube, phosphorene has open possibility of designing a sensitive gas sensor. Previous work as in reference [22] shows that this kind of nanomaterial have higher binding energy. Baierle et al. [22] indicate the introduction of carbon substitution increases the binding energy substantially. The material structure of graphene, phosphorene and carbon nanotube are almost the same with some common characteristic due to the hexagonal bond between the molecules. Likewise, BC 2 N, it has the same hexagonal structure. Considering that BCxN has been increasingly investigated [26,27], the BC 2 N compound was selected for the analysis in this study. Moreover, as BC 2 N has not been explored as a hydrogen gas sensor thus far, its analysis for H 2 gas sensing applications is essential. We are motivated to investigate the possibility of applying BC 2 N for use in sensing hydrogen gas. With this we can benchmarked the performance with other nanomaterials.
To determine the feasibility of carbon-doped boron nitride nanoribbon (BC 2 NNR) for sensing H 2 gases during temperature variations, including at room temperature, the performance reaction of H 2 was analyzed in this study considering different temperature settings. Herein, BC 2 NNR with armchair edges was used for sensing, and its electronic properties were investigated based on the band structure. The sensing ability was assessed by analyzing the adsorption energy, charge transfer, and sensitivity. The simulation results can form the basis for further experimental analyses on hydrogen gas sensors.
The remainder of this paper is organized as follows. The computational details of the simulation are described in Section 2. Section 3 presents the obtained results and discusses their inferences. Finally, the conclusions of the study are summarized in Section 4.

Methodology
The simulation was performed using QuantumATK from Synopsys (version S-2021-06). Carbon atoms were added within the structure of BN to obtain the BC 2 N structure (Fig 1) with different numbers of dimer lines (n), namely, 4, 6, 8, 10, and 12. Subsequently, four-unit cells representing length, L were repeated along the z-axis and maintained constant. The arrangement of B-N-C-C along the armchair lines in this study was based on a previously reported study [28]. Figs 2-4 depicts three additional models with different positions of H 2 molecules attached to 12-BC 2 NNR. In general, the H 2 gas molecules were attached to the top of BC 2 N over carbon (Fig 2), boron (Fig 3), and both boron and nitrogen atoms (Fig 4). Figs 5-7 depicts the structures obtained by multiplying the H 2 molecules in different positions. As indicated in   (Fig 6), and both carbon and nitrogen atoms (Fig 7). Generally, the structures of H 2 molecules illustrated in Figs 2-4 were considered for the analysis. All structures exhibited an initial bond length of 1.48 Å for A-BC 2 NNR. The H 2 molecules for the three models with different positions were randomly attached at varying distances to analyze the gas adsorption. For H 2 gas of the 12-BC 2 NNR structure located directly on the carbon, boron, and both boron and nitrogen atoms, the bond lengths were 1.13, 1.21, 1.92, and 1.96 Å, respectively. Furthermore, the distances for the addition of H 2 molecules were 1.16, 1.4, 1.16 and 1.17 Å, for carbon, boron and both carbon and nitrogen atoms, respectively. Finally, the distance of the H-H gas molecule was maintained at 0.64 Å.
All calculations were performed using the linear combination of atomic orbitals-based density functional theory. The exchange-correlation electron was computed using the Perdew-  Zunger formalism of local density approximation. Furthermore, the double-zeta polarized basis set was used for all calculations. To ensure numerical accuracy, the density mesh cut-off energy was set to 75 Hartree. The Brillouin zone (BZ) was sampled using a k-point sampling of 1×1×100 for the electronic structure calculation. Initially, the geometries of all atomic structures in the unit cells were optimized using the limited-memory Broyden-Fletcher-Goldfarb-Shanno algorithm optimization method. LBFGS is the recommended optimizer to use in QuantumATK. In addition, it is necessary to undergone geometry optimization as it will relax the structure and find the minimum energy structure in the modeled structure in order to obtain accurate results. A vacuum region of 10 Å was added along the non-periodic boundary to prevent artificial inter-ribbon interactions. Thus, the sensing performance of H 2 on BC 2 N was investigated during temperature changes to maximize its functionality. The temperatures selected for the evaluation were 298, 500, and 1000 K.

Results and discussion
The electronic properties were focused on the energy band structure, adsorption energy, and charge transfer. Meanwhile, the transport properties were focused on I-V characteristic to calculate the sensitivity. In the simulation of the energy band structure, A-BC 2 NNR followed the BZ route from Z point to Γ and back to Z point. Figs 8-10 depicted sample of the energy band   structure for T = 298 K. Table 1 lists the evolution of the energy band gap for the three temperatures considered in the study at three different positions of H 2 gas molecules attached to the surface of 12-BC 2 NNR; the simulation was performed to determine whether the energy band gap would alter.
The energy band gap was observed in the gap between the conduction and valence bands using the band structure process. According to Table 1, the energy band gap openings at all temperatures for the three different positions of H 2 gas molecules were similar (approximately 1.83 eV). The changes in the energy band gap were similar, indicating that the shift in the lower conduction band and the upper valence band were independent of the temperature changes. However, it can be noticed in Figs 8-10 above, each position of H 2 gas have difference number sub-band lines in both conduction and valence bands. Furthermore, the attachment position of the gas molecule was not a sensitive factor affecting the energy band gap of BC 2 N.
To further understand the potential of BC 2 NNR as a gas sensor, the adsorption energy, charge transfer, and sensitivity were simulated. The adsorption energy (E ad ) was calculated using Eq (1) to measure the strength of H 2 gas sensing bonds with BC 2 N [29,30].
where E(BC 2 N+H 2 ) denotes the total energy of BC 2 NNR with an absorbed H 2 gas molecule, E (BC 2 N) indicates the total energy of BC 2 NNR, and E(H 2 ) represents the total energy of the isolated H 2 gas molecules. The charge transfer (Q) was evaluated using the basic set method, as indicated in Eq (2), using the Mulliken population (N) [31][32][33].
where Q a and Q b denote the net Mulliken charges of a gas molecule after and before its adsorption, respectively. Figs 11 and 12 depict the comparison of the adsorption energy and charge transfer data, respectively, in terms of the attachment position of gas molecules at elevated temperatures, varying from 298 to 1000 K.
To analyze the gas sensing mechanism at room temperature (25˚C), 298 K was considered as one of the temperature values for the evaluation. Among the three positions, the strongest binding site was identified in the case of the hydrogen atom located directly above the boron atom (Fig 3), with an adsorption energy of −0.05091 eV. A significant transfer of electrons of 212.401 e was observed from H 2 gas molecules to BC 2 N. This is noticeable different on the charge transfer when hydrogen molecules on boron atom. According to the energy band structure in Fig 9 above, more localized states produced below Fermi level in position of boron atom compared to carbon and both boron and nitrogen atoms. This is indicated that large amount of charge acceptor of 212.401 e was accumulated at valence band state. Thus, this signify BC 2 N received a large number of electrons from the H 2 gas. As mentioned by Zhou et al. [24], charge transfer contributor should be larger in order for it to sensing. Minimum increments of 6.7 and 2.6% were observed in the adsorption energy and charge transfer,

PLOS ONE
Performance of BC2NNR as a hydrogen gas sensor at high temperatures respectively, from the positions of hydrogen on carbon and both boron and nitrogen atoms.  [23], where the adsorption energy from H 2 gas molecules to Ti 2 CO 2 was achieved at −0.05 eV and a charge transfer of 0.003 e was observed [23]. The negative sign of the adsorption energy indicated stronger adsorption and stability [32] with the exothermic reaction [34]. For the case of WS 2 the top of a W atom (Tw), has the most favorable site with −0.075 eV with smallest charge transfer contributions with

PLOS ONE
Performance of BC2NNR as a hydrogen gas sensor at high temperatures 0.002 e [24]. The substitution impurities of carbon atom in different sites of BNNT has shown physisorption occurs with no preference for H 2 molecule to absorbed as very close adsorption energies was recorded of −0.156 and −0.163 eV [22]. Apparently, result showing in carbondoped BNNT is larger than in presented work, BC 2 NNR. This could be due to the different in dimensional models of BN nanomaterial and to its chemical properties' reaction. Salih et al. [32] reported that a negatively charged value of charge transfer indicates the electron loss of the gas. In this study, a negative charge was observed only at a temperature of 298 K at both boron and nitrogen atoms (Fig 12).
At 500 K, the preferable binding site was observed between the hydrogen molecule and carbon atom, with an adsorption energy of 12.43445 eV and a charge transfer of 29.373 e from H 2 gas molecules to BC 2 N. Here, only infinitesimal increments of 0.01 and 0.06% were observed in the adsorption energy and charge transfer, respectively, from boron and both boron and nitrogen atoms. As indicated in Fig 11, the adsorption energies of two positions on BC 2 N were slightly greater than 12.43445 eV, ranging from 12.42642 to 12.43284 eV. This was also observed in the case of charge transfers between the H 2 gas molecules and BC 2 N, which were smaller than 29.373 e, ranging from 1.082 to 7.125 e. According to Salih et al. [32], the researcher found that the negative sign of the adsorption energy indicated stronger adsorption and stability. Thus, referring to the significant adsorption energy recorded, the adsorption energy observed for temperature of 500 K were seen as not stronger and stable, although the preferable binding was recorded. This can be realized that, the positive sign achieved for all three positions.
By contrast, the strongest binding site at 1000 K was identified at the position of boron with an adsorption energy of −0.05552 eV and a charge transfer of 8.495 e from H 2 gas molecules to BC 2 N. The adsorption energies of two positions (at carbon and both boron and nitrogen) on BC 2 N ranged from −0.05141 to −0.05449 eV, whereas the charge transfer ranged from 6.215 to 6.364 e, which were smaller than that obtained at the position of boron. The adsorption energy at the position of boron was increased by 7.4 and 1.9% compared to those at the positions of the boron and nitrogen and carbon atoms, respectively. The comparison of the elevated temperature changes from 298 to 1000 K (Fig 11) revealed that the adsorption energy was the highest for all three positions at 1000 K.
According to Guo et al. [35], the adsorption energy should be greater than −0.50 eV to effectively capture gas molecules on solid surfaces. As indicated in Fig 11, none of the adsorption energy observed in this study was greater than −0.50 eV. Additionally, Guo et al. [35] reported that the perfect h-BN sheet is unsuitable for gas separation and adsorption at room temperature. This could be the reason for the adsorption energy not effectively capturing the H 2 gas molecule on BC 2 N at 298 K. In addition, BC 2 N was seen as unsuitable for sensing H 2 gas due to its relatively small adsorption energy, approximately taken at −0.05 eV and low binding energy was reported between BC 2 N and H 2 gas. To further evaluate the potential of BC 2 N as a hydrogen gas sensor, the sensitivity was calculated using currents obtained from the current-voltage (I-V) characteristics with and without exposure to gas molecules, as indicated in Eq (3) [36].
where I and I O denote currents from the system with and without the H 2 gas molecules, respectively; both currents were calculated at the highest bias voltage of 3 V. Table 3 lists the comparison of sensitivity based on the attachment positions at temperatures varying from 298 to 1000 K. Fig 13 depicts the two electrodes which are left and right electrodes that connected to BC 2 N to evaluate its performance as a hydrogen sensor. The I-V characteristics before and after the adsorption of H 2 gas molecules were simulated considering the three temperatures. The construction of the two electrodes are implemented in QuantumATK software under plugin features tool called device from bulk. Initially, the nanoscale structure needed for QuantumATK for transport calculations consists of three main parts; left electrode, central region and right electrode as depicted in Fig 13. In a transistor perspective, the left and right bulk (periodic) electrodes are connected to the device's central region, which is finite (non-periodic) along the transport direction. The central region is also known as the scattering region because changes in the physical and chemical environment in the central region scatter charge carriers travelling between two electrodes. Furthermore, the central region determines the device's functionality, whereas the left and right electrodes are modelled as perfect leads connecting the device to the external source and drain.  As depicted in Fig 14, the I-V curves of BC 2 N exhibited a clear nonlinear behavior. Before the adsorption of the H 2 gas molecules, the current passing through BC 2 N was 8.74 μA for a bias voltage of 3 V at temperature of 298 K. After adsorption, the current for the bias voltage of 3 V at the three positions that were attached to the top of BC 2 N exhibited a considerable reduction. After the adsorption of H 2 gas molecules, the carbon and both boron and nitrogen atoms exhibited less change in current, which were 8.58 μA (1.83%) and 8.55 μA (2.17%), respectively. The highest current reduction of 18.54% (7.12 μA) was observed at the position of boron (Table 3). Therefore, the results verified that the detection of H 2 gas molecules by BC 2 N was not sensitive at room temperature.
For further evaluation, a certain concentration of H 2 molecules was added to BC 2 N under room temperature. The position of attachment was randomly selected at the top of carbon, boron, and carbon and nitrogen in BC 2 N (Figs 5-7). The corresponding I-V curves are plotted together shown in the inset of Fig 14 above. The adsorption of the added H 2 molecules affected the performance of BC 2 N as a hydrogen sensor. The molecules positioned on carbon and both carbon and nitrogen exhibited the highest currents with 9.83 and 9.61 μA, respectively, at a bias voltage of 3 V; the sensitivity was increased by approximately 12.47 and 9.95%, respectively. Although the H 2 molecules were added at different locations on the boron atom in BC 2 N, the I-V curves indicated a significant reduction in current to 6.79 μA (22.31%) when compared to other atoms. This implied that boron in BC 2 N was not sensitive for the detection of H 2 molecules, and hence not preferable.
As indicated in Fig 15, the I-V curves of BC 2 N exhibited a clear nonlinear behavior. A slight increase in current was achieved for the H 2 molecules positioned only on carbon when the applied bias voltage was greater than 2 V. According to the sensitivity of the three positions of H 2 molecules at 500 K (Table 3), the current passing through BC 2 N was 8.58 μA at a bias voltage of 3 V before the adsorption of H 2 gas molecules. However, owing to the adsorption of H 2 gas molecules located on the carbon atom, the current was increased to 8.75 μA by approximately 2% at a bias voltage of 3 V. A significant reduction in current was observed in the case of H 2 gas molecules positioned on boron; herein, the current was reduced to 6.97 μA, which was approximately 18.76%, at the same bias voltage of 3 V. Similar behaviors were observed with a current reduction of approximately 1.63% (8.44 μA) after the adsorption of H 2 gas molecules located on both boron and nitrogen atoms. Considering the I-V characteristics, the Furthermore, a certain concentration of H 2 molecules was added to BC 2 N at a temperature of 500 K. As depicted in Fig 15, the current at a bias voltage of 3 V was increased to 9.84 μA by 14.69% at both carbon and nitrogen atoms. This was followed by an increase in current for the molecule positioned on the carbon atom, with an approximate increase of 12.59% (9.66 μA). Conversely, the current in the case of the molecule positioned on the boron atom exhibited a comparatively lower increment to only 6.73 μA (21.56%).
Finally, H 2 gas molecules detection was tested at a temperature of 1000 K; Fig 16 depicts the I-V characteristics before and after the addition of molecules. The trend of I-V curves at 1000 K was similar to that observed at 500 K. The I-V curves of BC 2 N exhibited an apparent nonlinear behavior. The current passing through BC 2 N before adsorption of H 2 gas molecules was 8.59 μA at a bias voltage of 3 V. The I-V curves of the adsorbed H 2 gas molecules located on the carbon atom indicated that the current was marginally increased to 8.88 μA (3.38%) compared to that before adsorption. This was followed by a slight reduction in current to 8.48 μA (1.3%) on both boron and nitrogen atoms. The highest reduction in current of 2.46 μA (71.36%) was observed for the H 2 molecule positioned on the boron atom. The I-V curves of H 2 gas molecules on boron is not depicted in Fig 16 owing to the significant reduction in current.
To further maximize the potential of BC 2 N as a gas sensor, a certain concentration of H 2 molecules was added to BC 2 N at a temperature of 1000 K; Fig 16 depicts the corresponding I-V curves. After H 2 molecules were added, carbon and both carbon and nitrogen atoms recorded the highest currents of 9.88 μA (15.02%) and 9.67 μA (12.57%), respectively, at a bias voltage of 3 V. However, the added concentration of H 2 molecules to boron resulted in a major current reduction of 6.78 μA (21.07%); this indicated that the position on boron was not sensitive, because the current passing through BC 2 N before adsorption of H 2 gas molecules was 8.59 μA.

Conclusions
In this study, the sensing mechanism of hydrogen gas on BC 2 NNR was investigated using computational simulations. Three different temperatures were considered for the analysis; the same trend was observed in all cases, wherein the strongest binding site was achieved at high adsorption energies with high charge transfers. In the case of elevated temperatures of 298 and 1000 K, the preferable binding site was at the boron atom, whereas, for a temperature of 500 K, the preferable binding site was observed at the carbon atom. Based on the analysis of the I-V characteristics observed at the three temperatures, the adsorption of H 2 gas molecules located on boron atoms was determined to be the least sensitive; particularly, the sensitivity was the lowest (71.36%) at 1000 K. After the adsorption of H 2 gas molecules at room temperature, BC 2 N as a sensor was no longer sensitive. However, further addition of H 2 gas molecules at room temperature exhibited a remarkable increment in sensitivity. Herein, the sensitivities of H 2 gas molecules located on carbon and both carbon and nitrogen atoms increased by 12.47 and 9.95%, respectively. Furthermore, the trends of I-V curves were identical at elevated temperatures of 500 and 1000 K before adding more concentrations of H 2 gas molecules. The highest sensitivity was observed in the carbon atom, whereas the boron and both boron and nitrogen atoms were not sensitive. Moreover, after adding further concentration of H 2 gas molecules, the carbon and both carbon and nitrogen atoms exhibited dominant sensitivity at all elevated temperatures. The most insensitive detection of BC 2 N as a hydrogen sensor before and after adding concentrations of H 2 gas molecules was identified on the boron atom.